It is established practice in the cementing of liner or casing in a well bore to drill a well bore of large diameter to a certain depth and then line the drilled well bore with a casing which is cemented in place, which is sometimes called surface or intermediate casing. Thereafter, a smaller size bit is used and a continued extension of the borehole is made into the earth formations and a liner is cemented in place in this continued extension of the borehole. Thus, a borehole is comprised of a series of borehole sections of decreasing diameter and a string of pipe is cemented in place in each borehole section and overlaps the bottom end of an adjacent liner or casing.
To cement the liner or string of pipe in place, the liner, which is a tubular member, is lowered through the cemented casing in the borehole and suspended by a liner hanger in the bottom end of the casing next above the borehole section receiving the liner. Between the liner and the drilled borehole is an annular space or borehole annulus. A cement a slurry is pumped down a tubing string and through a connection with the liner hanger around the bottom of the liner and into the borehole annulus. After the cement slurry is properly positioned in the borehole annulus and hardens, the cement provides a seal against fluid migration vertically in the borehole annulus and permanently affixes the liner in the borehole. There may be a number of liners utilized in a well bore depending upon the depth to which the well is being drilled. As in most drilling operations, the drilling of the well bore is conducted with a mud or liquid which contains various materials such as weighting materials which provide the hydrostatic pressure control as the drilling operation is conducted so that the hydrostatic pressure of mud in the well bore is always calculated to balance, approximately any expected pressure that may be encountered by gas or oil formations traversed by the borehole. The use of the drilling mud thus provides a control against blowout of the well and the consequence hazards therefrom. The main job of the drilling mud, besides formation control is to return the drilled cuttings to the surface.
In the cementing of a liner in a borehole annulus during the drilling operation, the liner has a float shoe at its lowermost end which is broadly described as a closure member with orifices to permit the flow of fluids from the bottom of the liner into the annulus between the liner and the borehole and a one way check valve to prevent reverse flow of cement back into the liner. To set the liner in another pipe in the well bore, a liner hanger is typically provided at the top of the liner. The liner hanger has a system for providing a releasable connection with a setting tool attached to a string of tubing extending from the earth's surface. After setting the liner hanger in the next above string of pipe in the borehole, a cementing string of pipe is utilized to inject a cement slurry into the liner and then flowing through the float shoe on the liner where the cement slurry is forced upwardly in the borehole 35 annulus between the liner and the well bore and displaces the drilling fluid or mud in the borehole annulus towards the earth's surface until such time as the top of the slurry of cement in the borehole annulus is properly positioned.
Because the size of the liner and the size of the borehole are known factors, it is possible to calculate the volume of cement slurry necessary to fill the borehole annulus between the liner and a well bore to a proper level. It is also common in introducing the cement slurry to provide isolation of the cement slurry between the mud in the borehole and between the fluid used to drive the cement slurry down the string of pipe. The isolation devices also wipe the mud from the wall of the cementing string of pipe so that mud in the pipe does not contaminate the cement slurry. The isolation devices or means used in the cementing string of pipe or tubing to a liner hanger are called pump down plugs. The pump down plugs have elastomer cup members for wiping the wall of the string of tubing and for permitting the application of mud fluid pressure to the slurry of cement, while separating the cement slurry from borehole fluids or mud.
To this end, as in the prior art, there have been provided on a tubular member (usually in the end of the string of pipe) upper and lower tubular cementing wiper plugs which are releasably yet independently attached to the tubular member. The cementing wiper plugs have outer elastomer cup members for wiping the wall of the liner. The cementing wiper plugs are respectively adapted to receive one of the pump down plugs and respectively form an isolation assembly means in the liner.
At spaced apart locations near the casing shoe, there are upper and lower wiper plug catchers which trap or catch the respective upper and lower cementing wiper plugs.
In the operation, a first cementing pump down plug is inserted into the tubing string at the surface of the earth and is followed by the slurry of cement which pushes the cementing pump down plug downwardly through the tubing string and the drilling mud in front of the pump down plug is displaced. When the first cementing pump down plug latches into the bore of the lower cementing liner wiper plug, the pump down plug and wiper plug act as a single movable seal assembly to close off the cross section of the liner and also serve to wipe the walls of the liner while moving toward the float shoe. Thus, there is a minimum of intermix of the slurry of cement with the drilling mud and the mud on the pipe wall is also removed preventing it from contaminating the cement. When the calculated volume of the slurry of cement necessary to fill the borehole annulus between the liner and the borehole has been placed in the tubing string, a second follow-up pump down plug is disposed in the string of tubing to provide a single moving, sealing plug member behind the column of the slurry of cement in the string of tubing. Another fluid such as drilling mud is supplied to provide the driving force on the upper pump down plug behind the column of cement. When the second or upper pump down plug engages and locks into the bore of the upper cementing liner wiper plug on the tubular member, the upper pump down plug and liner wiper plug assembly travel downward behind the column of cement in the liner. Thus, there are sealing assembly devices which are forward and rearward of the column of cement slurry in the liner and travel downwardly with the column of cement until the lowermost wiper plug assembly engages the lower plug catcher located in the liner above the float shoe. A valve in the cementing shoe is open during the pumping operation permitting the cement slurry to enter through the float shoe into the borehole annulus between the liner and the borehole. Cement slurry then is moved up in the borehole annulus as the rearward cementing plug assembly travels downwardly until the rearward plug assembly reaches a second plug catcher in a collar in the liner and locks in position in the landing collar, thus trapping the cement slurry in the borehole annulus between the liner and the borehole.
The problem with prior art systems is premature release of the cementing wiper plugs on the tubular member in the liner hanger and/or faulty valve operation at the cementing float shoe when the cementing plug assembly reaches the lower catcher in the cementing float shoe.
For example, in U.S. Pat. No. 3,364,996 there are spaced apart upper and lower wiper sleeves or cementing wiper plugs releasably secured to a tubular member on a liner hanger. The upper liner wiper plug has a first smaller diameter bore relative to the bore of the string of pipe or tubing while the lower liner wiper plug has a still smaller bore. The first pump down plug preceding the cement slurry in the tubing string engages the smaller bore of the lower liner wiper plug and releases the lower liner wiper plug from the tubular member so that the assembly can move downwardly through the liner in front of the cement slurry until it reaches a landing collar or catcher above the cementing float shoe. The second pump down plug following the cement slurry in the string of tubing next engages the bore of the upper liner wiper plug and releases the upper liner wiper plug from the tubular member to move the assembly downwardly in the liner and where it ultimately locks in an upper landing collar above the cementing float shoe.
When the first pump down plug and lower liner wiper plug reach a landing collar, the landing collar has a sleeve valve which is part of the cementing float shoe where the sleeve valve is opened to admit or bypass the cement slurry to the borehole annulus.
Another type of dual cementing plug system is shown in U.S. Pat. No. 3,635,288 where the first pump down plug engages a smaller diameter bore in the lower liner wiper plug and where the pump down plug and the lower liner plug define a built in valve structure. The upper liner wiper plug has a bore matching the bore of the running in string and provides latching fingers which spring into latching position in the bore to catch the second cementing dart.
In both of the foregoing devices, a major problem of premature release of the two liner wiper plugs occurs when a differential pressure is introduced across the plugs during cementing the neither the bypass valve in the cementing float shoe nor a bypass valve operated by a cementing float shoe have been reliable.